I. Field of the Invention
This invention relates generally to cardiac rhythm management devices, including pacemakers and automatic implantable cardiac defibrillators, and more particularly to a safety feature that comes into play when it is desired to disable the runaway protection feature of such devices.
II. Discussion of the Prior Art
For many years now, implantable cardiac pacers and pacer/defibrillators have incorporated a runaway protection capability that prevents the device from pacing the heart at an inordinately high rate due either to a software or a hardware fault. A runaway pacemaker is a pacemaker malfunction that may occur in single-chamber or multi-chamber pacing systems. It is usually the result of a minimum of at least two separate component failures within the pulse generator. The result is the rapid delivery of pacing stimuli to the heart, with the potential for inducing lethal arrhythmias, such as ventricular tachycardia or fibrillation. Newer devices incorporate a runaway protect circuit that prevents stimulation above a preset rate, typically between 180 and 200 bpm. Although somewhat rare, with modern pacing devices, this represents a medical emergency. Prompt surgical intervention to replace the device or, if all else fails, cutting the leads must be performed. This is most serious for a patient who may be pacemaker dependent.
As those skilled in the art appreciate, in antitachycardia pacing, it is often desirable to apply high frequency bursts of stimulating pulses in an attempt to induce an episode of tachycardia so that the capabilities of the device in terminating the episode can be evaluated. This requires that the runaway protection logic of the device be disabled, thus allowing stimulation at rates above the runway protection limit. However, disabling the runaway protection feature of a cardiac rhythm management device is inherently unsafe to do. Should the firmware lose control of the process due to a hardware or a firmware fault, the device may operate without runaway protection for an indefinite amount of time.
Accordingly, there is a need for a way to more safely disable the runaway protection feature of an implantable cardiac rhythm management device that will insure that it is automatically re-enabled within a set time following its being disabled. The present invention provides a logic arrangement for achieving this end.
A cardiac rhythm management device in accordance with the present invention comprises an R-wave sensor for sensing ventricular depolarization events, a P-wave sensor for sensing atrial events and a pulse generator for applying cardiac stimulating pulses to the heart of a patient. It further includes a programmed microprocessor-based controller that is coupled to the R-wave sensor to the P-wave sensor and to the pulse generator for providing stimulating pulses to the heart at times determined by the microprocessor-based controller. The microprocessor-based controller incorporates a programmable upper rate limit for normally precluding the pulse generator from applying the stimulating pulses to the heart at a rate greater than the established upper rate limit. The software, executable by the microprocessor-based controller, permits selective disabling of the runaway protection limit. In further accordance with the present invention, means are provided that are responsive to the selective disabling of the runaway protection limit and for automatically re-enabling the runaway protection limit a predetermined time following the disabling of the runaway protection limit.
In accordance with a further feature of the invention, this last mentioned means preferably comprises a first timer comprising a counter for counting regularly occurring clock or pacing pulses and for producing a runaway protection re-enabling signal when a predetermined time interval represented by the count value in the first counter reaching a predetermined value has elapsed. Under software control, however, the first counter may be reset before the first predetermined count value is reached to thereby extend the time interval that the runaway protection remains disabled. A second timer, also a counter for counting regularly occurring clock or pacing pulses is used to produce a signal to re-enable the runaway protection upon reaching a second predetermined count value, irrespective of the count state of the first counter. In this fashion, the length of the interval during which fast pacing may take place can be preset and will automatically end if either the first counter or the second counter reaches its predetermined count.
There are, of course, additional features of the invention that will be described hereinafter which will form the subject matter of the appended claims. Those skilled in the art will appreciate that the preferred embodiments may readily be used as a basis for designing other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions since they do not depart from the spirit and scope of the present invention. The foregoing and other features and other advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.